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tetrapods within a single clade, the tetrapodomorph
sarcopterygians.
The transition from water to land among animals
is now well supported by informative fossils for
vertebrates, but frustratingly less so for arthropods
and molluscs. The actual processes involved in
this evolutionary transition remain largely elusive.
As for vertebrates, at any rate, the fin-limb transition
as currently outlined by palaeontologists is roughly
consistent with the recent data provided by develop-
mental genetics. The latter are far less informative,
however; we understand which morphogenetic
processes may have been involved but, to date,
only fossils provide factual support for intermediate
conditions (Coates et al. 2007).
been centred about a somewhat finalist conception
of a conquest that culminates with land vertebrates,
mammals and man; who finally survives due to the
simultaneous terrestrial evolution of all other organ-
isms which he depends on. A botanist (or a palaeo-
botanist) could say the same, considering how much
seed plants are dependent on animals to disperse,
reproduce and survive (Hall´ 2002). Apart from
spermatophytes, however, land plants could in prin-
ciple exist in the total absence of animals (as long as
bacteria and fungi are there).
Terrestrialization is in fact the result of indepen-
dent, but sometimes remotely contingent, events.
Similar crucial events in the history of life have
occurred earlier, such as the rise of photosynthesis,
motile benthic metazoans, predation, larval devel-
opment (or metamorphosis) and the planktonic life
in metazoans; that is, what we now call 'marine
ecology' (Butterfield 2007; Peterson et al. 2007).
Nowadays, molecules and fossils concur in pro-
viding us with a more consistent timing of the
history of life. We are certainly a long way from
the time when we will have a reliable picture of
the very deep past of life on Earth, but we are
aware of some key events in the past 2 Ga or so.
We also understand the mechanisms that underlie
the patterning of eukaryotic life for nearly 1 Ga,
until the rise of 'animals' (Peterson et al. 2007).
The rise of terrestrial ecosystems, however fas-
cinating they may be for palaeontologists, is com-
parable to other preceding dramatic changes in the
aquatic world.
Terrestrialization today
The term 'terrestrialization' appeared in the
mid-19th century but currently encompasses many
different adaptive processes, depending whether it
is considered by for example, anatomists, palaeon-
tologists, ecologists or physiologists. Terrestrializa-
tion is in fact a transition between water and another
fluid: air. For most of the plants and animals that
underwent terrestrialization, the substrate remained
basically the same from sea, river or lake bottom to
land. What changed were essentially gravity, the
mode of oxygen intake, water retention and conduc-
tion, sometimes the mode of locomotion (for
animals) and the protection against ultra-violet
rays. Among these constraints, water retention was
the most difficult to overcome because an aquatic
organism is merely an osmotically isolated part of
its watery environment; that is, a bag of water in
water. A terrestrial organism, however, is a bag of
water in air.
In common with all examples of evolutionary
transition, the border between non-terrestrialized
and terrestrialized organisms is not clear-cut.
There are many instances of anatomical structures
and functions that were once regarded as signatures
of terrestrialized organisms, but are in fact exapta-
tions of homologues which were perfectly func-
tional in fully aquatic organisms. The tetrapod
limbs is a classical example, but its selective advan-
tage would be very obscure if it did not arise in con-
junction with other anatomical or physiological
innovations such as the choanae (internal nostrils).
The latter turned up some tens of million years
earlier, in fully aquatic forms (Clack 2002).
Similar conjunctions of chronologically discon-
nected but adaptively consistent characters are
also found in arthropods and embryophytes.
The emphasis on vertebrates when dealing with
terrestrialization in popular science gives the
impression that all ideas regarding this event have
Selection, adaptation and evolutionary
radiations
Current research in integrative palaeobiology
throws some light on questions that have long
been puzzling evolutionary biologists, namely the
rise of phenotypic innovations and the biological
processes that underlie major evolutionary radi-
ations. The latter are generally conceived as the
result of the selection of a specific character which
happened to occur in a particular environmental
context and soon ensured a selective advantage to
a large number of species. In the case of terrestriali-
zation, common knowledge suggests that almost all
metazoan and metaphyte taxa were potential candi-
dates for the conquest of land, in a context of
increasing levels of atmospheric oxygen. In fact,
as well as soil bacteria and some unicellular organ-
isms, only four major clades comprise most of the
fully terrestrialized life: fungi, embryophytes,
arthropods and vertebrates. Annelids and gastropods
are generally overlooked in the usual narrative of
terrestrialization and there are conjectures about
why,
for example,
there is no terrestrialized
